1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a storage medium and, more particularly, to an image processing technique for correcting a luminance of an overlap region of projection screens projected by a plurality of image display apparatuses.
2. Description of the Related Art
Conventionally, when a multi-screen display is configured using a plurality of projection type image display apparatuses, an overlapping image region (image overlap region) is assured between neighboring projection type image display apparatuses, and an image signal of the image overlap region undergoes luminance correction, thus obscuring a seam. By setting the image overlap region to have a predetermined width, even when the projection type image display apparatuses have slightly different characteristics of luminance levels, colors, and the like, such differences are not easily visually recognized.
As a luminance correction method, a method using electrical signal processing (electrical luminance correction) is available. When a luminance level is to be lowered linearly toward an image end portion in an overlap image region, in consideration of a display gamma (γ), an output image signal Vo is calculated by:Vo=((x/W)^γ)*Vi  (1)where Vi is an input image signal, W is an image overlap region width, and x is a distance from an image end of the image overlap region. When the image overlap region width is variable, since the circuit implementation of this calculation requires a large circuit scale, a method also using an LUT is generally used (Japanese Patent Laid-Open Nos. 2005-117266 and 2007-295026).
A technique disclosed in Japanese Patent Laid-Open No. 2005-117266 reduces a circuit scale by implementing (1/W) in equation (1) by multiplication of a coefficient read out from the LUT and a right bit shift operation. However, the number of coefficients stored in the LUT has to be equal to a maximum W of the variable widths. Also, a luminance correction LUT corresponding to the display gamma (γ) is additionally required, and a circuit scale reduction effect is consequently low.
On the other hand, in a technique disclosed in Japanese Patent Laid-Open No. 2007-295026, letting Wb be a reference image overlap region width, for example, a luminance correction coefficient Tb(x) which considers the display gamma (γ) is stored in a reference LUT. The luminance correction coefficient Tb(x) is given by:Tb(x)=(x/Wb)^γ (x=0, 1, . . . , Wb)  (2)
Then, when the image overlap region width is set to be Ws, an LUT value as a luminance correction coefficient Ts(x) is referred to, as given by:Ts(x)=Tb(x*Wb/Ws) (x=0, 1, . . . , Ws)  (3)When the image overlap region widths satisfy Ws≦Wb, a position in the image overlap region width Ws is associated with that in the reference image overlap region width Wb to have one-to-one correspondence. However, when Ws>Wb, since a position in the image overlap region width Ws cannot be associated with that in the reference image overlap region width Wb to have one-to-one correspondence, an appropriate luminance correction coefficient cannot be obtained in the overlap region, and the overlap region is recognized as a level difference. FIG. 5 shows an example of a distribution of luminance correction coefficients Ts in the related art. The ordinate plots values of luminance correction coefficients Ts, and the abscissa plots the overlap region widths. FIG. 5 shows a luminance correction coefficients Ts (501) when Wb=256 with respect to Ws=100, and a luminance correction coefficients Ts (502) when Wb=32. By setting Wb to match a maximum value of the variable image overlap region width, Ws≦Wb can always be satisfied. However, the number of coefficients to be stored in the LUT increases in correspondence with the magnitude of Wb.